linux-kernel - Re: [PATCH] vma: Detect infinite loop in vma tree

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Message-ID: <d0174131-1fc5-46d2-af87-a42a72a31487@suse.cz>
Date: Thu, 31 Oct 2024 18:06:57 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] vma: Detect infinite loop in vma tree

On 10/31/24 18:01, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> There have been no reported infinite loops in the tree, but checking the
> detection of an infinite loop during validation is simple enough.  Add
> the detection to the validate_mm() function so that error reports are
> clear and don't just report stalls.
> 
> This does not protect against internal maple tree issues, but it does
> detect too many vmas being returned from the tree.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Jann Horn <jannh@...gle.com>
> ---
>  mm/vma.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index 68138e8c153e..60ed8cc187ad 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -615,7 +615,8 @@ void validate_mm(struct mm_struct *mm)
>  			anon_vma_unlock_read(anon_vma);
>  		}
>  #endif
> -		i++;
> +		if (++i > mm->map_count)
> +			break;

Would it make sense to allow some slack so that the error below can
distinguish better between off-by-one/few error from a complete corruption?

And in that case assign some special value to "i" (-1?) to make it clear
this was triggered?

>  	}
>  	if (i != mm->map_count) {
>  		pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i);